1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) display, and in particular to an active-matrix organic light emitting diode (AM-OLED) which increase display life.
2. Description of the Related Art
Organic electroluminescent devices or organic light emitting diode (OLED) displays have the characteristics of self-emission and can be arranged in a matrix without requiring a backlight module. Organic light emitting diode (OLED) displays are thin and light-weight, and also have the advantages of high contrast, high resolution, low power consumption, and wide viewing angle. Due to these advantages, it is expected to that OLEDs will be adopted as the next generation of display devices.
Generally, an active-matrix organic light emitting diode (AM-OLED) display is driven by electric current to provide illumination. FIG. 1 is a circuit configuration scheme of a pixel unit in a conventional active-matrix organic light emitting diode (AM-OLED) display. Referring to FIG. 1, the AM-OLED display pixel unit comprises an organic light emitting diode 1, a switch transistor T1, a driving transistor T2 and a capacitor 2, wherein the transistors T1 and T2 are Thin Film Transistors (TFTs).
As shown in FIG. 1, a display signal “data line” connects the drain of the switch transistor (TFT) T1, and a scan data signal “scan line” connects the gate to switch the switch transistor T1 on and off. Furthermore, a voltage drive source V+ connects the drain of the driving transistor T2 and the source is connected to the anode of an organic light emitting diode 1. A capacitor 2 is coupled between the sources of the transistors T1 and T2. The capacitor 2 can be charged keeping a hold voltage to enable the driving transistor T2 such that a current passes through the driving transistor T2 to drive the organic light emitting diode 1 provide illumination.
As mentioned above, an active-matrix organic light emitting diode (AM-OLED) display requires adequate current passing through the driving transistor T2 to drive the organic light emitting diode 1. Long term use, however, leads to deterioration of the electrical characteristics. Specifically, the threshold voltage increases when current passes through the driving transistor T2 and leads to device degradation. Therefore, after long term use the driving current will degrade such that the illumination and life time of the organic light emitting diode 1 decrease.
As shown in FIG. 1, only a driving transistor T2 is used to drive the organic light emitting diode 1 in a pixel unit of the conventional active-matrix organic light emitting diode (AM-OLED) display. A disadvantage to the current structure is that when the driving transistor T2 is turned on for an extended period of time, the temperature of the driving transistor T2 increases while the threshold voltage decreases due to the heat generated by current continuously passing through the driving transistor T2. Therefore, this pattern of increasing current, temperature, and heat ultimately cause the driving transistor T2 to fail.
To overcome the above mentioned disadvantages, the present invention provides an active-matrix organic light emitting diode (AM-OLED) display with increased life.